1. Field of the Invention
The present invention relates generally to signal processing, and more particularly to a system and method for reducing crosstalk between adjacent channels in a wavelength division multiplexing (WDM) transmission.
2. Related Art
In optical communications, wavelength division multiplexing (WDM) techniques are often used to transmit multiple digital signals along a single optical transmission medium. An example of such an optical transmission medium is a fiber optical cable, such as those used in the telecommunications industry. WDM transmissions comprise multiple signals or channels, each having a different and distinct wavelength (or a particular range of wavelengths). In this fashion, multiple signals are simultaneously transported along a single fiber optical cable.
At the receiving end of a WDM transmission, the individual channels are reconstructed using optical filtering techniques that serve to separate the channels according to their wavelengths. Typically, 2, 4, 8 and 16 individual channels are multiplexed in this fashion.
In a typical example, a long distance telecommunications carrier may transport a WDM transmission having 16 individual digital communication channels along a single fiber. Each channel typically comprises a signal having the capacity to carry a large number of simultaneous voice and data channels. For example, each channel in the WDM transmission may comprise a standard SONET Optical Carrier-48 (OC48) channel having a transmission rate of 2.4 Gbps.
In this example, each OC-48 channel has the capacity to carry approximately 32,256 standard voice signals. Thus, because 12 OC48 channels are multiplexed in the fashion described above, a single optical fiber can carry the equivalent of approximately 387,072 standard voice channels using WDM techniques.
Accordingly, WDM techniques allow for a dramatic increase in the signal density of new and existing fiber optical cables. It is desirable to multiplex as many channels as possible using such WDM techniques in order to increase the transmission capacity of a service provider without having to deploy additional and/or new fiber optical cables.
However, problems occur as the signal density of WDM transmissions increase. One problem likely to occur as a result of increasing the number of channels in a WDM transmission, is the problem of crosstalk between adjacent channels. This type of crosstalk occurs when components of separate WDM channels interfere and combine with adjacent WDM channels. The term "adjacent" as used herein refers to channels that are adjacent to each other according to their wavelength in the optical WDM transmission.
As stated, crosstalk is more likely to occur as more channels are crowded into the limited passband of an optical link. One technique used to avoid or reduce the instances of crosstalk between channels is to precisely control the wavelengths of the individual channels and to restrict the modulation bandwidth of each channel. However, this type of control places stringent requirements upon lasers and in particular, it places stringent requirements upon optical filters at the receive end of the WDM transmission.
For example, the use of highly selective and precise optical filters at the receive end of a WDM transmission would yield a reduction in the unwanted crosstalk components from adjacent channels. However, the use of such filters has several disadvantages. One disadvantage is that they are more expensive than less precise filters. Another disadvantage is that they are more susceptible to frequency drift. In addition, such filters can have undesirable delay characteristics.
Another technique that has been used to reduce and/or avoid crosstalk is to provide a guardband between adjacent channels. In this fashion, leakage from one wavelength to another is thereby reduced due to the spacing between the channel frequencies. However, such guardbands take up valuable bandwidth space in the optical link and such space is wasted because useful information is not transmitted within the guardbands.
Therefore, what is needed is an efficient and inexpensive system and method that can be used to reduce crosstalk between adjacent channels in a high density WDM transmission.